Allow grades app to be zero-migrated

lms/djangoapps/grades/migrations/0005_multiple_course_flags.py

@@ -18,3 +18,17 @@ class Migration(migrations.Migration):
             field=CourseKeyField(max_length=255, db_index=True),
         ),
     ]
+
+    def unapply(self, project_state, schema_editor, collect_sql=False):
+        """
+        This is a bit of a hack. This migration is removing a unique index that was erroneously included in the initial
+        migrations for this app, so it's very likely that IntegrityErrors would result if we did roll this particular
+        migration back. To avoid this, we override the default unapply method and skip the addition of a unique index
+        that was never intended to exist.
+
+        The assumption here is that you are never going to be specifically targeting a migration < 0005 for grades,
+        and will only ever be migrating backwards if you intend to go all the way back to zero and drop the tables.
+
+        If this is not the case and you are reading this comment, please file a PR to help us with your intended usage.
+        """
+        pass